Method for the Positioning of Operative Cylinders on a Grinding Machine and Grinding Machine Which Implements This Method

ABSTRACT

A method for positioning operative cylinders, for example of rolling-mills or paper-mills on a grinding machine, includes at least the following steps: A—positioning the transversal, longitudinal center-line plane of the cylinder in correspondence with a reference point situated between a movable piece-holder head and tailstock center of the machine; B—moving the piece-holder head and the tailstock center in the direction of the cylinder until there is a reciprocal coupling. The present invention relates also to a machine for implementing the above method.

FIELD OF THE INVENTION

The present invention relates to a method for the positioning ofoperative cylinders on a grinding machine and a grinding machine whichimplements this method; it should be pointed out that the definition“operative cylinders” refers, for example, to cylinders of rolling millsfor the rolling of metals, those used in paper-mills, those used inoil-pressure applications, shafts of marine motors and in general allcylinders made of metallic and non-metallic materials which are ofindustrial interest.

BACKGROUND OF THE INVENTION

The grinding of the cylinders described above, and in particularcylinders of rolling-mills or paper-mills, has various drawbacks,associated first of all with the encumbrance and weight of the samewhich are often considerable.

The latter, in fact, have dimensions varying from 40 mm to 2300 mm indiameter, 1,000 mm to 12,000 mm in length and weights ranging from a fewtens of daN up to 230-250 t.

The function assigned to these cylinders requires that they be processedwith a precision and accuracy in the order of a few thousandths of amillimeter.

The grinding machines which can be used for these cylinders thereforehave specific characteristics and must not be confused with the smallerand traditional lathes or similar machines which, in this sense, are notcapable of effecting these grinding processes.

The grinding of a cylinder must always be preceded by a preliminarypositioning phase of the latter on the machine, which is effected byinserting the cylinder to be ground between a piece-holder head and atailstock center and subsequently supporting it between centers of themachine and, if necessary, tightening it with one or more lunettes orcontrast devices, or resting the cylinder itself directly on a pair ofsupporting lunettes; either of the two solutions is selected in relationto the type of cylinder to be ground and the processing to be effected.

In the case of the processing of so-called “thin” cylinders, forexample, or cylinders in which the diameter/length ratio is particularlysmall (a typical example of these is represented by cylinders of“Sendzimir” rolling mills or, more generally, “cluster” rolling mills),the known art envisages that the cylinder to be ground is first loadedonto the grinding machine with a loading device (such as, for example, agantry crane, a jib crane or an automatic loading device), it is thenmoved until one of its ends is supported by the center of thepiece-holder head, the tailstock center is then moved until the contactwith this supports the opposite end of the cylinder, and finally thelunettes or contrast devices are moved along the cylinder, so as toposition them correctly with respect to the part of the cylinder to beground.

The lunettes or contrast devices are normally produced as supportssuitable for opposing the thrusts that the grinding wheel exerts on thepiece being processed; they are normally positioned at regular distancesalong the cylinder, in order to uniformly oppose the forces during thegrinding process.

As the piece-holder head is fixed, all the important distances aretherefore measured from the piece-holder head and the tailstock centerand lunettes or contrast devices are positioned in relation to thisreference, moving them in an axial direction to move them towards oraway from the reference, depending on the length of the cylinder.

The execution of such operations, typically defined by the term set-upcan be manual or automatic. In the former case, the set-up time isextremely important as a series of various operations are required,which must be accurately effected in order to verify their correctness.In the latter case, a reduction in the set-up times jeopardizes thesimplicity and reliability of the machine, as both the lunettes and thetailstock center must be equipped with suitable movement devices andrelative control systems.

Another drawback linked to the known art relates to the fact that duringthe loading, the cylinder must be moved in three directions and inparticular, among these, also along the longitudinal axis, in order tobe correctly positioned on the grinding machine; these regulationmovements require the use of a loading device. In manual plants, thisdevice is typically a gantry crane or a jib crane and the precision withwhich the cylinder must be moved and positioned requires longpositioning times, skilled operators and the risk that, with anerroneous maneuver, the cylinder, or machine, or both, can be damaged.In automatic plants, the movement of the cylinders is effected with3-axis loading systems and if, on the one hand, they reduce themaneuvering times and increase the safety of the same, on the other,they require machines which move the load along 3 axes and areconsequently, by nature, complex, costly and require maintenance. Thepositioning of the lunettes and tailstock center is also quite lengthywith the traditional technology, in the sense that they must be movedalong the cylinder to be correctly positioned, care being taken tomeasure the distances from the side of the cylinder associated with thepiece-holder head. Also in this case, if, on the one hand, the manualsolution is economical, on the other, it requires time and accuracy onthe part of the operator, whereas although the automatic solutionguarantees operational velocity and precision, it requires a movementand control mechanism for the tailstock center and for each of thelunettes.

In the case of manual machines, these problems are even more serious dueto the fact that cylinders having different lengths must often be groundconsecutively: these set-up operations must therefore be effected foreach processing, with considerable time consumption.

In particular, not only must the tailstock center be moved each time,but also the lunettes, as the distances from the piece-holder head(which serves as a reference) must be measured each time and all thelunettes must therefore be moved along the cylinder.

SUMMARY OF THE INVENTION

The general objective of the present invention is consequently toovercome these and other drawbacks of the known art.

This objective is achieved by a grinding method and a grinding machinehaving the characteristics specified in the enclosed independent claimsand in the dependent claims, which should be considered as being anintegral part of the present description.

A positioning method has been conceived for these cylinders whichenvisages at least the following steps:

positioning the transversal, longitudinal center-line plane of thecylinder in correspondence with a reference point situated between amovable piece-holder head and tailstock center of the machine;

moving both the piece-holder head and the tailstock center until theyare coupled with the cylinder.

The movement of the piece-holder head and tailstock center is preferablyof the same entity, in the same direction and opposite senses.

In this way, the reference system is identified specifically by thetransversal, longitudinal center-line plane of the cylinder, and notalready by one of its ends, as in the known art.

For this purpose, the piece-holder head and tailstock center areintegral in their translation movement with respect to the frame of themachine, for example by coupling them both with the same screw, so thata rotation of the latter causes the same movement of both, in the samedirection but in opposite senses.

In this way, the advantages obtained are evident: firstly, it is notnecessary to axially move the cylinder on the machine to position it onthe piece-holder head and it is therefore sufficient to use a loadingdevice with only two movement axes, simpler and less costly, whether itbe manual or automatic.

Furthermore, the central reference system, coinciding with thelongitudinal center-line plane of the cylinder, offers other advantages:if, for example, an odd number of lunettes (one, three, etc.) is used,it is not necessary to reposition the lunette or central contrast deviceeach time, which is preferably fixed on the machine in the position inwhich the center-line of the cylinder is assembled.

Similar advantages are also obtained even if the number of lunettes isequal (two, four, etc.) as, also in this case, the positioning of thelunettes must be symmetrical with respect to the center-line of thecylinder.

This advantage in terms of set-up time saving is verified in eachprocessing and even more so when various consecutive processings ofcylinders having different lengths must be effected on the same machine:in this latter case, in fact, it would be necessary in the known art toalso reposition the possible central lunette each time, whereas with thepresent invention, it always remains in the same position.

By using the reference system situated in the center-line of thecylinder, moreover, an additional advantage is obtained: experience hasshown that it is not in fact necessary to move the lunettes when thedimension of the cylinders that must be consecutively processed on thesame machine varies within a range of about 0.5 meters.

It has been noted, in fact, that if the variation in the longitudinaldimensions of the cylinders remains within this range, all the lunettes(both the central one, if envisaged, and the side ones) can be kept inthe same positions, thus drastically reducing the set-up time.

Furthermore, the positioning of the side lunettes is simplified as thedistances can be measured by the central reference system, i.e. thecenter-line of the cylinder simplifying the movement and avoidingpossible errors.

BRIEF DESCRIPTION OF THE DRAWINGS

The structural and functional characteristics of the invention, as alsoits advantages with respect to the known art, will appear more evidentform the following description, referring to the enclosed drawings whichshow a possible practical embodiment of the same invention.

In the drawings:

FIG. 1 illustrates a machine according to the present invention;

FIG. 2 illustrates a detail of the machine of FIG. 1;

FIGS. 3 and 4 illustrate details of the machine of FIG. 1.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Describing first the positioning method, object of the presentinvention, it always comprises the following steps:

A—positioning the transversal, longitudinal center-line plane of thecylinder in correspondence with a reference point situated between amovable piece-holder head and tailstock center of said machine;

B—moving both the piece-holder head and the tailstock center until theyare coupled with the cylinder and supporting it.

An overall version of the positioning method also envisages thefollowing steps:

A1—preparing a rolling cylinder to be ground;

A—positioning the transversal, longitudinal center-line plane of thecylinder in correspondence with a reference point situated between amoveable piece-holder head and a tailstock center of the machine,

B—moving both the piece-holder head and the tailstock center until theyare coupled with said cylinder and supporting it;

C—positioning at least two supporting side lunettes or contrast devicesof said cylinder at the same distance from said central contrastlunette.

The reference point mentioned in step A above can possibly coincide withthe positioning of the transversal, longitudinal center-line plane ofsaid cylinder in correspondence with a fixed lunette with respect to theframe of the machine and/or with respect to the side lunettes.

Numerous variants can obviously be applied to the method described, allobject of the present invention; in this sense, some of the steps of themethod can be inverted (for example steps B. and C. can be exchanged) orother steps of the method, relating to secondary regulations of themachine can be added; or it is also possible that the central lunette,due to a particular form and geometry of the cylinder, may not be used.

With reference to step A, the reference point, this is preferably fixedonto the machine and coincides with a lunette or contrast device, inparticular with the central lunette, as will appear more evident below,in the description of the machine. This lunette can obviously be removedto allow the grinding of cylinders having a particular form andgeometry.

Both the piece-holder head and tailstock center can preferably be movedso as to be always equidistant from the reference point: their movement,in other words, is of the same entity and is always effected in the samedirection but in opposite senses.

Also with respect to the side lunettes, or those situated at the sidesof the central lunette which coincides with the reference point, thesecan be advantageously moved to the same degree and in the samedirection, but in opposite senses. There can be one or more of theseside lunettes but always in the same quantity on both sides with respectto the central lunette.

In this way, the advantages discussed above are obtained, as thereference point for the whole positioning phase is no longer an end ofthe cylinder but its center-line, thus allowing the above advantages tobe obtained.

As far as the machine is concerned, reference should be made to FIGS. 1and 2.

The grinding machine 10 comprises a base 11 on which a piece-holder head13 and tailstock center 12 are movably assembled, which serve to moveand possibly support a cylinder 14 which is positioned between themduring the grinding operation.

More specifically, the machine 10 is of the type in which the processingcylinder is supported by two rotating centers 81 and 83 positioned inthe piece-holder head 13 and tailstock center 12.

The lunettes 20, 21, 22 are positioned in contact with the cylinder andhave the function of providing a contrast to the possible bending of thecylinder due to the same processing.

It should be pointed out that the machine 10 can equivalently be of adifferent type, in which the lunettes, in addition to having thefunction described above, also have the function of supporting the wholeweight of the cylinder being processed, whereas the piece-holder headand tailstock center only have the function of bringing it intorotation, to allow its grinding.

The piece-holder head 13 comprises a single body which is capable oftranslating thanks to the presence, in the lower part, of two linearguides 80 with which it is coupled by means of suitable sliding blocks.

The rotating center 81 can be housed inside the body of the piece-holderhead 13, which, in some applications, forms a rest for the cylinder. Inthe rear part, on the other hand, a motor 71 can be positioned, with therelative transmission, for rotating the cylinder.

The tailstock center 12 also comprises a body which is capable oftranslating thanks to the presence of two linear guides 80 situated inthe lower part of the structure with which it is coupled by means ofsuitable sliding blocks.

The movable center 83 can be housed inside the tailstock center 12,which, in some applications, forms a second rest for the cylinder.

Furthermore, if required by the particular application underexamination, the tailstock center 12 can house a device in its lowerpart, which generates a preloading through a spring device.

This preloading acts in an axial direction and guarantees the continuouscontact between the cylinder and two movable centers 81 and 83 and, atthe same time, the desired axial preloading, if this is required by theprocessing, as is the case, for example, in the grinding of so-called“thin” cylinders.

The machine 10 also comprises a grinding wheel which can be movedparallel to the axis of the cylinder, not shown for the sake ofconciseness, and in any case known in itself in this type of grindingmachine.

Both the piece-holder head 13 and tailstock center 12 can be movedlinearly along the base 11 so that they can move towards or away fromeach other, to house cylinders having different lengths.

The piece-holder head 13 and tailstock center 12 are coupled to movementmeans which allow their linear approach or withdrawal movement to thesame extent, in the same direction and in opposite senses.

The movement means of this example comprise a worm screw 16 engaged withboth the piece-holder head 13 and tailstock center 12, so that, whenbrought into rotation, the screw 16 actuates both the approach orwithdrawal in the same direction along which the screw 16 extends; thesemovement means are illustrated in more detail in FIG. 2.

For this purpose, the screw 16, produced in a single piece, has twothreaded parts 160 and 161 with which a corresponding female screw ofthe piece-holder head 13 and tailstock center 12 is engaged.

In order to allow a movement in the same direction and of the sameentity, but in opposite senses, the two threaded parts 160, 161 of thescrew 16 have contrasting helicoidal threadings, for example ananticlockwise threading and a clockwise threading, preferably having thesame pitch.

It should be pointed out that two separate helicoidal screws with acontrasting threading can be equivalently envisaged, instead of a singlescrew 16, or the latter may not be in a single piece but separable intotwo different sections connected with appropriate joints.

The screw 16 is actuated in rotation by a motor 17, for example anelectric brushless motor or the like.

In this way, by correctly positioning the cylinder so that itstransversal longitudinal center-line plane is in correspondence with thereference point of the machine 10 and actuating the screw 16, thepiece-holder head 13 and tailstock center 12 move towards the ends ofthe cylinder, subsequently being coupled with it and possibly supportingit.

It should be noted that this solution also effects a kind ofself-centering of the cylinder between the head 13 and tailstock center12, if in fact the cylinder were not correctly positioned along itsaxial direction, as both the head 13 and tailstock center 12 move withrespect to a common central reference to the same degree, in the samedirection and in opposite senses, their approach movement would allow acentering of the cylinder.

This particular feature, on the one hand makes manual loading of thecylinder simpler, safer and more rapid, whereas on the other, itsignificantly facilitates the projecting of mechanisms of a possibleautomatic loading system.

The machine 10 also comprises contrast or supporting lunettes 20, 21,22.

These, in the particular case of the grinding of so-called “thin”cylinders, must at least be present in one unit 20, but generally in atleast three units 20, 21 and 22 and normally in an odd number. The useof lunettes in an even number, i.e. without the central lunette, is onlyenvisaged in particular cases, when the geometry of the cylinderrequires this. Consequently, except for this particular case, the set oflunettes always comprises a central lunette 20 or, more generally, thecentral lunette 20 and two side lunettes 21 and 22, or four sidelunettes etc.

The cylinder is assembled with its transversal, longitudinal center-lineplane (which, as in the case of so-called “thin” or Sendzimir cylinders,often coincides with the symmetry plane passing through the center ofgravity and perpendicular to the generatrices of the cylinder) in aposition corresponding to the central lunette 20.

The latter, at least in the case mentioned above, is consequentlygenerally fixed in position with respect to the machine 10 or at itsbase destined for resting on the ground, and does not have to be movedwhen cylinders having different lengths are being processed, as, in anycase, the longitudinal center-line point of the cylinder is alwaysassembled in correspondence with said central lunette 20.

In this sense, it is also possible to mechanically fix the centrallunette 20 to the machine 10, for example by means of screws or bolts,or make it integral with at least part of the base 12, so that itrepresents a central and privileged reference point for the measurementand calculation of other dimensions.

Considering the case, for example, of three lunettes, as far as the sidelunettes are concerned, 21 and 22, their position can be manuallyregulated or, in more evolved variants of the machine 10, they can beconnected to a specific worm screw or to the same screw 16 whichactivates the piece-holder head 13 and tailstock center 12, so that theyare moved in the same direction and to the same degree, but in oppositesenses, by means of a single and simple actuation of the screw withwhich they are coupled, consequently reducing or eliminating the risksof an incorrect positioning of both: in this way, in fact, they cannotbe positioned at different distances from the central lunette 20, butare always advantageously equidistant with respect to the same.

In this sense, a variant envisages controlling the position of both thelunettes 20, 21, 22 and the piece-holder head 13 and tailstock center 12by means of a single screw command and a series of threaded sleeves 90(one for each device to be moved) shown in FIGS. 3 and 4 enclosed.

Each sleeve 90 comprises a cylindrical mantle and is engaged with theworm screw 16 by means of a threading inside the same mantle.

Each sleeve 90 can also be connected to the unit to be moved by means ofa suitable mechanism. In principle, it should be noted that, as analternative to the brake blocks described hereunder, in the light of thedisclosures provided herein, an expert in the field can also envisageother different solutions; for example, instead of a brake block, theblocking means of the unit to be moved can comprise electromagnets,mechanical coupling means such as fixing pegs, wedges, removableinterference profiles or other similar items.

For illustrative and non-limiting purposes, one of thesecoupling/decoupling means is described hereunder.

The coupling/decoupling means of the sleeve to the part to be positionedcomprise two arms 91, 92 pivoted to the structure of the part to bemoved (piece-holder head, tailstock center and lunette) and eachprovided with an engagement end 93, 94 with said sleeve 90 and anopposite free end 95, 96 coupled with an actuator 97.

The actuator 97 is preferably a linear actuator, for example a gas,pneumatic, hydraulic, electric actuator or similar: in the non-limitedembodiment illustrated, it is a hydraulic actuator equipped with acylinder and piston which protrudes variably from the cylinder.

The actuator 97 is coupled, preferably hinged, to the free ends 95, 96of the arms 91, 92; each engagement end 93, 94 of each arm 91, 92 isequipped with a clamp with linings for coupling with the outercylindrical surface of the sleeve 90.

The two arms 91, 92 have their fulcrum along their body in the points O1and O1 1 so as to oscillate, so that as the two free ends 95, 96approach, the opposite ends 93, 94 move away and vice versa.

The machine 10 preferably comprises a single screw 16 and both the head13 and the tailstock center 12 and lunettes 20, 21, 22 are equipped withthese coupling/decoupling means, each having a specific correspondingsleeve.

In this way, there is a plurality of sleeves along the body of thescrew, one for each coupling/decoupling means.

The functioning can be deduced from what has been described so far: whenthe actuator of one of the coupling/decoupling means is activated toclose the linings over the corresponding sleeve, the same part of themachine (head, tailstock center or lunette) can be controlled intranslation as a result of the rotation of the screw 16, by activatingthe latter, in fact, the sleeve, which is prevented from rotating, movesalong the screw 16, entraining in translation the element to which thearms are pivoted.

It is therefore possible, with a simple and accurate system, to movehead, tailstock center and lunette to position them correctly on thebase 11, drastically reducing the set-up times of the machine 10.

Furthermore, in this way, both the lunette and the head and tailstockcenter can be activated contemporaneously or separately with a singlecommand, obtaining an extremely high versatility of the machine.

In short, the connection between each female screw and the part to bemoved associated with it, is obtained by means of a system whichoperates in the following way: if a certain part is to be moved, forexample the tailstock center, said blocking system constrains thethreaded sleeve 90 (or female screw) to the body of the tailstock centeritself and, at the same time, prevents its rotation, so that, onceactuated, the screw rotates and the female screw, constrained to thetailstock center, cannot rotate integrally with the screw andconsequently causes the tailstock center to translate.

If, on the contrary, the piece-holder head is to be moved and thetailstock center kept immobile, it is sufficient to disengage thespecific sleeve from the body of the tailstock center andcontemporaneously constrain that of the piece-holder head; in this way,once the screw begins to rotate (in this case to move the piece-holderhead) the sleeve of the tailstock center will rotate integrally withitself, maintaining the sleeve itself in the same axial position andconsequently not causing any movement of the tailstock center.

In this way, either the lunettes or the head or the tailstock center canbe simply and rapidly activated contemporaneously or separately, toallow the cylinder to be suitably positioned and supported.

There can therefore be various movement conditions of the parts, whichshould be considered as being an integral part of the method describedabove, for example:

1) Symmetrical positioning of the lunettes, piece-holder head andtailstock center with respect to the central reference of the machine:if only the female screw of both lunettes is integral with the screw,they will move for the same distance but in opposite senses, due to theconstructive form of the worm screw. Contemporaneously the female screwsof the piece-holder head and tailstock center are disengaged from thescrew and these two groups consequently remain motionless. Afterpositioning the lunettes, the piece-holder head and tailstock center arepositioned by making the female screws of these integral and disengagingthose of the lunettes. This positioning procedure can be useful, forexample, in the case of the grinding of symmetrical cylinders (forexample Sendzimir).

2) Asymmetrical positioning of the piece-holder head and tailstockcenter-symmetrical positioning of the lunettes: in this case, thelunettes are synchronously positioned, making both of the female screwsof the latter integral with the screw. In this way, by actuating thescrew, they will move for the same distance but in opposite senses.After positioning these two groups, their female screws are disengagedfrom the screw. At this point, the female screw of the piece-holder headand tailstock center are alternatingly made integral with the screw,moving first one group and then the other.

3) Asymmetrical positioning of the lunettes, piece-holder head andtailstock center with respect to the center-line of the cylinder: inthis case, the groups are positioned, each time making the female screwof one group integral with the screw, whereas the female screws of theother groups not involved in the movement, are disengaged. Thisprocedure can be very useful for moving the above parts in the case ofthe grinding of asymmetrical cylinders (such as typically hot and coldrolling cylinders), or in applications where the piece-holder headremains fixed such as, in general, all applications except for thegrinding of Sendzimir cylinders.

Numerous alternatives to the machine 10 described so far are possible,all considered as being an integral part of the present invention.

The movement means for the head 13 and for the tailstock center 12and/or for the side lunettes 21 and 22, for example, can be produceddifferently, for example by means of a transmission belt coupled withtrolleys, so as to enable the movement of the head 13 and tailstockcenter 12 and/or lunettes, as envisaged above.

The movement means can equivalently comprise two separate screws and twomotors, even if, in this case, the cost of the machine is higher.

The center-line of the whole machine 10 can also advantageously coincidewith the central lunette 20, making the positioning of the cylinder evenmore simple and intuitive.

The objectives mentioned in the preamble of the description havetherefore been achieved.

The scope of the invention is defined by the following claims.

The invention claimed is:
 1. A positioning method for operativecylinders, for example of rolling-mills or paper-mills, on a grindingmachine the method comprising the following steps: a. positioning atransversal, longitudinal center-line plane of a cylinder incorrespondence with a reference point situated between a movablepiece-holder head and tailstock center of a grinding machine; and b.moving said piece-holder head and said tailstock center towards adirection of said cylinder until there is a reciprocal coupling, forentraining and/or supporting the cylinder.
 2. The method according toclaim 1, wherein said reference point is positioned equidistant fromsaid piece-holder head and said tailstock centercentre.
 3. The methodaccording to claim 1, wherein said moving in step b comprises movingsaid piece-holder head and said tailstock center of same entity, in asame direction and in opposite senses.
 4. The method according to claim1, further comprising the following steps: preparing the cylinder to beground; positioning the transversal, longitudinal center-line plane ofthe cylinder in correspondence with the reference point situated betweenthe moveable piece-holder head and the tailstock centre of the machine;moving said piece-holder head and said tailstock center until coupledwith said cylinder; and positioning at least two supporting sidelunettes (21, 22) of said cylinder at a same distance from saidreference point.
 5. A grinding machine for operative cylinders, forexample of rolling-mills or paper-mills, comprising: a base; apiece-holder head; and a tailstock center suitable for being coupled atopposite ends of a cylinder to be ground, in order to rotate saidcylinder, wherein said piece-holder head and said tailstock center aredesigned to be translated with respect to said base.
 6. The grindingmachine according to claim 5, wherein said piece-holder head and saidtailstock center are coupled with a moving system configured to allowtheir linear translation for their reciprocal approach or withdrawal. 7.The grinding machine according to claim 6, wherein said moving system isconfigured to allow a movement of said piece-holder head and saidtailstock center in a same direction, of a same entity and in oppositesenses.
 8. The grinding machine according to claim 6, wherein saidmoving system comprises an endless screw coupled with the piece-holderhead and tailstock center.
 9. The grinding machine according to claim 6,further comprising a central lunette for central contrast and at leasttwo side lunettes for one or both of contrast or support of saidcylinder, said central lunette being positioned in an equidistantposition from said piece-holder head and said tailstock center.
 10. Thegrinding machine according to claim 9, wherein said moving system forthe piece-holder head and the tailstock center is coupled with said sidelunettes, for determining a reciprocal movement in a same direction, ofa same entity and in opposite senses.
 11. The method according to claim4, wherein said reference point coincides with a supporting lunette ofsaid cylinder.
 12. The grinding machine according to claim 8, whereinsaid screw preferably is composed of a single piece and comprises twothreaded parts having a discordant, helicoidal threading.